The unidentified gunmen entered a nightclub in the city of Monterrey late Monday, opening fire on the staff and clients of the Men's Club Matehuala, said Jorge Domene, a Nuevo Leon state security spokesman.

Among the dead was a man who tried to escape through the roof of the bar and fell to his death into a neighboring establishment, Domene said.

Northern Mexico is among the areas most affected by drug cartel violence as police crack down and rival gangs attack one another for control of lucrative smuggling routes.